1. Field of the Invention
This invention relates generally to gearing arrangements and, more particularly, toward gearing arrangements for driving a load.
2. Prior Art
Worm gears driving a gear wheel are known. Such an arrangement typically has a gear face, i.e. the distance along the crest of each tooth, with a fixed diameter. Thus, each gear wheel has the shape of a flattened cylinder with teeth extending from vertical sides. The contact area 41 between a worm gear 40 and a gear wheel 42 under light loads is shown in prior art FIG. 1.
When a substantially increased load is applied to worm gear 40 and gear wheel 42, such as shown in prior art FIG. 2, there is reduced tooth contact area 41A. Such reduced contact area 41A may cause gear stripping or other damage to the gear train. It would be desirable to reduce or eliminate the separation of the drive worm gear and the gear wheel caused by large loads.
In particular, these problems can occur in an automotive windshield wiper system. The gear train must be designed so it can withstand the windshield wiper motor stall and armature inertia shock loads under adverse situations such as when the wiper blades are frozen to the windshield or under heavy snow loads where the wiper blade wipe angle is restricted. Under such conditions, the gear teeth will experience high stress levels and may fail in service. These are some of the problems this invention overcomes.
Also known are gear wheels with teeth having extensions at both ends in order to follow the contour of the worm gear. Such double ridges with an intermediate depression for receiving the worm gear present a difficult tooling problem. When molding the gear wheel of a plastic material, the tooling must be formed of at least two parts to permit removal of the formed gear wheel. High quality gear wheels are difficult to obtain because of the possibility of a mismatch between the tool parts. Furthermore, such gear wheels typically have solid interior portions which do not provide for flexing. When there is no flexing all the driving load of the worm gear is applied to the gear wheel tooth in contact with the worm gear and may cause failure of the gear wheel tooth.